In certain technical fields, for example, in connection with space travel there is a need for extremely light weight and precise containers which at the same time must be of high strength construction. The walls of such containers are made of fiber material, which are wound about a winding core and the spaces between the fibers are filled with a suitable bonding material, as is well known in the art. For example, glass fibers or carbon fibers are used in this connection and the bonding material is usually a synthetic resin, for example, an epoxy resin. Such containers require for their production a relatively stiff, that is, dimensionally precise and dimensionally stable winding core in order to assure the required high precision in the shape of the containers.
Heretofore such containers have been manufactured by first producing sheet metal containers with relatively thin walls, for example, of titanium or aluminum. The sheet metal container served as core onto which the fibrous material has been wound, whereby the sheet metal core became a permanent part of the container wall forming a bond with the fibrous material. Leaving the metal core inside the container has certain disadvantages. On the one hand the metal core increases the weight which is unpermissible in many instances. On the other hand, the metallic inner surface of the container limits the use of the container in an undesirable manner, for example, where the container content would attack the inner metal wall.
In order to avoid the above disadvantages of prior art containers of this type attempts have been made to employ divisible cores which may be reused. Such cores comprise a plurality of individual segments which are held in proper position relative to each other by spacer means. The fiber material is then wound onto the core which is subsequently disassembled in the finished container and the individual core elements or components are removed through end openings or access openings of the container. Such cores by their very nature are extremely expensive and in addition their usefulness is limited because it is not possible to vary as desired the ratio between a core segment and the diameter of an axis or end opening of the container. These openings may be relatively small and it is economically not feasible to separate the core into a large number of sufficiently small segments, especially since such cores are very precise tools which lose their dimensional precision after having been used several times, because resin fragments may stick to the core segments and such resin fragments must be removed prior to reusing the core segments. Thus, this prior art method of employing segmented reusable cores is rather expensive and generally not satisfactory.
In another prior art method so called lost cores were produced, for example, of quartz sand or hollow glass beads. After completion of the container wall, that is, after setting and, if desired, after finishing the outside of the container wall, the lost core would be destroyed and removed through the container openings. Such method is also very expensive because several cores must be produced for each container until the core finally has the desired dimensional or shape precision. An additional disadvantage of this method resides in the fact that a relatively high percentage of rejects must be taken into account, which also increases the costs.
Another disadvantage of the just described prior art methods is seen in that it is rather difficult to integrate into the wall structure of the container any wall elements, for example, inner reinforcing ribs or flanges forming access openings at the ends of the containers. Such containers in the course of their use are subject to very high internal pressures. Therefore, any structural elements forming an integral part of the wall structure of the container must be overlapped by the high strength composite wall material.